1. Field of the Invention
The present invention relates to a transmission in an outboard motor having a drive shaft which couples an engine in an upper part and a propeller in a lower part.
2. Description of the Related Art
Conventionally, among outboard motors in which an engine as a driving force source disposed in an upper part of an outboard motor body and a propelling device having a propeller disposed in a lower part are coupled to each other via a drive shaft, there are ones in which a transmission is provided in an appropriate middle position of the drive shaft. The transmission is shifted according to the traveling state of a boat having such an outboard motor, to an environment, or the like, so as to improve power performance, fuel consumption performance, and so on of the outboard motor.
Various types are devised as a specific structure for such transmissions. For example, in an outboard motor described in Patent Document 1, an automatic transmission made up of two planetary gears, three multiple wet clutches, and one one-way clutch is provided on a drive shaft coupling an engine and a propelling device (lower unit). According to this example, by setting the position of the transmission to a substantially middle portion in a vertical direction of the outboard motor, an outboard motor with a transmission can be realized compactly without affecting the profile of the entire outboard motor, thereby achieving both acceleration performance and fuel consumption performance.
Moreover, in one described in Patent Document 2, a two-speed transmission is constituted of parallel-axis spur gears which have high power transmission efficiency. Further, there are known one in which a switch point of the transmission is set by using a centrifugal clutch, one in which a transmission case and a water pump case are constituted integrally as in Patent Document 3, and the like.
Further, as in Patent Document 4, there is known one in which a transmission is provided in an intermediate part of a drive shaft, and an input shaft to the transmission and an output shaft from the transmission are coaxial.
Patent Document 1: Japanese Laid-open Patent Publication No. 2009-149202
Patent Document 2: Japanese Examined Patent Application Publication No. 03-14273
Patent Document 3: Japanese Examined Utility Model Application Publication No. 04-27757
Patent Document 4: Japanese Laid-open Patent Publication No. 62-191297
In one of Patent Document 1, the multiple wet clutches need a strong hydraulic device, which is expensive and for which energy needed for operating a hydraulic pump to maintain hydraulic pressure is large, becoming a cause of hindering fuel consumption performance. Further, although the multiple wet clutches couple smoothly, unlike the case of a four-wheeled vehicle, this function is not needed as much as in the case of a four-wheeled vehicle because changes in propeller speed of a propeller which only has small inertial moment are absorbed in the outboard motor. Accordingly, for the multiple wet clutches, a merit of alleviating shift shock is small with respect to high price, large weight, and large operating energy. Besides this, the planetary gears are expensive, and moreover, they are inferior to parallel-axis spur gears with respect to motive power transmission efficiency, and the like. From these points, it goes without saying that they are not suitable for outboard motors.
Further, a shift change mechanism in one of Patent Document 2 is a mechanical link mechanism, and is not able to suppress a shock transmitted to the link when it is shifted. Then, the position at an intermediate point is tolerated at a time of shift transition, and thus there is a problem of wear due to a relative speed difference. Moreover, it is set to a low speed side at a time of motive power direct coupling or to a high speed side at a time of via counter. Thus, while cruising which largely affects fuel consumption, it is motive power transmission via counter, and the fuel consumption worsens by the amount of gear transmission efficiency.
Moreover, in one of Patent Document 3, a counter shaft is disposed in a front side in a traveling direction, and a counter gear is housed in a gear case of forward tapered type, which is advantageous in terms of hydromechanics. Thus, a large-diameter gear cannot be disposed, causing a strength-related problem. Further, gear shift is performed by the mechanical link mechanism, and hence there is a problem that a shock at a time of speed shift is transmitted as is to the link side, and the like.